I've only been to one big one that was mostly cabaret-style floor show, with everyone on the dancefloor in the interval and afterwards. No one was very dressed up; smart casual and bring your hip scarf. But I've heard of others where everyone gets all their gear on.
And I've been to big shows with dancing afterwards where there have been some non-performing audience members in full-on bedlah. :shok:
I think 'Hafla' in Arabic means a party or celebration, but when I see the word 'Hafla', I cannot help but think of halva, the tahini based stuff that resides in my fridge.